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topic 54589

Yellow chemical conversion on Al turns PINK when exposed to vacuum


March 18, 2010

Hello,

I'm R&D designer working with miniature optical systems. We have made Al 6061-T6 parts with "yellow" chemical conversion coating Mil-DTL-5541 [link is to free spec at Defense Logistics Agency, dla.mil]F Type 1 Class 1A. They are used in vacuum chambers at room temperature (not cryogenic). The parts are always handled with gloves in class 10 000 clean room.

We have found that after as little as 24 hours of high vacuum (10-6 Torr) the parts loose their bright yellow and gradually take a pinkish color! The longer they stay, the pinker they get!
I know the Mil standard doesn't specify color as an acceptance criteria. I would have accepted the parts with the "pink" look from the shop (NADCAP certified for this process). It's just that the coating changes over time that worries me. Note that parts that stayed at normal pressure are still fine.

Is there any good reason to observe this color change phenomena on chem conversion coating?
We are worried that this could be indication of contamination or migration of components, or could it be some sort of corrosion?

Thanks in advance !

Patrice Cote
product ddesigner - Quebec City, QC, Canada



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